There have been numerous prior attempts at providing cementitious systems to meet the needs of the construction industry, particularly in the protection, waterproofing, and repair of concrete structures. The optimum system should set within a relatively short period of time into a hard mass or coating that has sufficient strength, abrasion resistance, and corrosion resistance. It is also highly desirable that these systems possess impermeability to fluids, particularly aqueous solutions. Also, such systems should not undergo excessive hardened volume changes under either wet or dry conditions.
For commercial use, these types of cementitious systems must also possess good bonding characteristics to damp or dry surfaces, early as well as long term strength, and practical field workability. They should be capable of withstanding freezing and thawing, as well as the action of salts, solvents and other corrosive substances. Although there have been a number of cementitious mixtures that possess one or more of the above-described desirable properties, none of the prior art to date has been able to achieve all of the foregoing in one composition and previous attempts have had only limited success. U.S. Pat. No. 4,357,166 discusses some of the limitations of these prior art compositions in columns 2 and 3.
While mixtures of Portland cement, calcium aluminate cement and gypsum have been in use for years, various problems concerning their use have been encountered.
Previous compositions containing other hydraulic cements such as aluminous cement and gypsum have been shown to exhibit long term wet expansion. For example, U.S. Pat. No. 4,357,166 discloses a cementitious composition which, when mixed with water, is capable of setting rapidly to a hard mass of high compressive strength without substantial shrinkage during setting and which exhibits reduced wet and dry volume changes in the hardened state. That invention also possesses a degree of impermeability to fluids along with abrasion, erosion, and chemical resistance, as well as other characteristics which are desirable in a composition having commercial usefulness in the construction industry. The advantages of that invention are achieved by a cementitious composition comprising a mixture of an aluminous cement, a gypsum, a drying shrinkage inhibitor, and a wet expansion inhibitor. The compositions of that invention preferably use Portland cement as a drying shrinkage inhibitor and a lithium salt as the wet expansion inhibitor. However, various accelerators, retarders and other admixtures, when added to aluminous cement and gypsum compositions, can significantly affect the hardened volume change (wet or dry), thus limiting their usefulness.
Additionally, previous attempts to blend gypsum with other hydraulic cements, while producing the desired effect such as fast-setting or reduced shrinkage, have also affected the hydraulic cement's soundness, durability, workability, resistance to water or wet/dry cycling stability or permanence of the resulting cement.
U.S. Pat. No. 4,045,237 discloses a cementitious composition which, when mixed with water, is capable of setting into a hard mass in a short period of time without substantial shrinkage during setting and early hardening and possessing a high degree of impermeability to fluids. The composition comprises a particulate admixture of calcined gypsum, high alumina cement and portland cement, or it may comprise a mixture of particles of high alumina cement and pressure calcined gypsum, without the use of Portland cement.
Portland cement consists mainly of tri-calcium silicate and dicalcium silicate. To prepare this compound, two types of raw materials are usually required--one high in calcium content, such as limestone or chalk, and the other rich in silica, such as clay or shale. These raw materials ordinarially contain an appreciable concentration or iron-bearing compounds. The presence of these compounds during the heating process leads to the formation of a clinker containing several percent of iron oxide.
The structure of this Portland cement clinker will, therefore, vary considerably due to variations in the composition and particle size of the raw materials as well as inconsistencies in the burning conditions, which leads to variations in clinker porosity as well as differences in crystalline sizes and forms found in the aggregates of crystallites.
Thus, the Portland cement component of the cementitious compositions disclosed in U.S. Pat. Nos. 4,357,166 and 4,045,237 contains an appreciable quantity of iron oxide as described above. In U.S. Pat. No. 4,045,237 for example, it is critical to employ between 0.1-10% of Portland cement in the mixture. The addition of greater than 10% of Portland cement leads to difficulties with the cement hydration reactions due to the resulting proportion of ferric oxide in the Portland cement-calcined gypsum-high alumina cement mixture.
U.S. Pat. No. 4,157,263, which is a division of the application leading to the issuance of U.S. Pat. No. 4,045,237 discloses a method for using the compositions claimed in U.S. Pat. No. 4,045,237 for use in repairing and waterproofing concrete structures and for filling voids and holes to form stable underpinnings or foundations for machinery or heavy equipment. The Portland cement utilized in the performance of this method contains, as noted above, appreciable quantities of ferric oxide, which restricts the amount of Portland cement which may be added to the mixture, thus reducing the strength of the composition.
Applicants have discovered that the hardened volume changes which occur in concrete, grout, patching material and water-proofing material made from blends of calcium sulfate hemihydrate, calcium aluminate cement and Portland cement can be reduced by using Portland cement containing reduced levels of iron oxide. Applicants have further determined that such a reduction of the iron oxide used in the Portland cement component dramatically increases the compressive strength of this composition and improves its bonding capacity to previously set concrete.
The maximum iron oxide content may vary, but as a general principle it should be a maximum of about 2 weight percent. Preferably, the iron oxide content should be as low as possible, but as a practical matter, amounts lower than 0.1 weight percent are difficult to achieve.